The present invention relates to a method and apparatus for non-destructively testing steel or other metal conduits to determine the existence and location of flaws.
In a great many applications, it is desirable to be able to examine a metal conduit for flaws or other imperfections which may not be visible to the eye either because of their size or because they are located on the interior wall of the conduit and therefore cannot be seen without internal examination. Non-destructive testing of pipes or conduits is also advantageous to determine if couplings or fittings are seated properly. For example, when assembling a conduit system for use in electromagnetic pulse hardening of electrical, electronic or communications wiring and cabling, it is necessary to determine if couplings are properly tightened and provide good shielding.
One class of apparatus for detecting flaws in metal objects comprises impressing on the object an alternating magnetic field which induces eddy currents which in turn produce their own magnetic fields. The presence of faults or irregularities in the object cause asymmetry or nonuniformity in the eddy current created magnetic fields which can be detected by means of suitable magnetic detectors such as Hall-effect devices. In U.S. Pat. No. 3,944,911 to Tornblom, the induced field rotates about the axis of the conduit and asymmetry in the detected magnetic field indicates a fault.
In a number of the prior art devices, a single magnetic sensor is utilized and must be moved over the surface of the object to detect and locate any faults which might be present. Alternatively, the conduit could be rotated past a stationary sensor. Such relative rotation between the magnetic field sensor and conduit creates a number of problems, however, especially when it is not possible to rotate the conduit. If the sensor must be rotated, electrical brushes or the like are required to provide contact between it and electrical processing apparatus. Furthermore, rather complicated mechanical drive mechanisms would be required to rotate the detector sensor in an arcuate path around the conduit. It is also necessary to assure that the path followed by the sensor is concentric with the conduit since the intensity of the magnetic field is often a function of distance from the conduit in a radial direction.
Although prior art devices are known which comprise a plurality of magnetic sensors positioned in a circular array within or outside a conduit for the purpose of flaw or joint detection, processing of the signals from the individual detectors is often complex and inprecise as to the exact location of the flaw. U.S. Pat. No. 3,166,710 to Schmidt and U.S. Pat. No. 3,843,923 to deVries et al are examples of conduit testers employing a static array of magnetic sensors.